Gosha, you may eventually find it useful to conduct your own basic research, rather than asking here, or guessing and hoping. Self-sufficiency can be learned, and would be significantly faster than posting and waiting for a reply here.

For instance, FAQ section 14.4, called "How is swap handled?", would have given you a great deal of useful information. Here is a relevant excerpt:

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14.4.2 - Swapping to a partition
On OpenBSD, the 'b' partition of the boot drive is used by default and automatically for swap.
No configuration is needed for this to take place. If you do not wish to use swap on the boot disk,
do not define a "b" partition. If you wish to use swap on other partitions or on other disks, you
need to define these partitions in /etc/fstab with lines something like:

/dev/sd3b none swap sw 0 0
/dev/sd3d none swap sw 0 0

And it further goes on to describe swapctl(8) and other ways to define and create swap areas.

Had you been interested enough to want to learn for yourself how the OS manages swap at boot up, you might have looked at the /etc/rc script started by init(8) on boot. It contains this command:

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swapctl -A -t noblk

Examining the swapctl man page, you might have learned both confirming and clarifying info about the boot disk swap space, as well as what that swapctl command actually does:

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Note: The initial swap device (root disk, partition b) is handled auto-
matically by the kernel and does not need to be added to /etc/fstab or
added via swapctl. It will show up as "swap_device" in the output dis-
played with the -l flag.
.
.
.
-A This option causes swapctl to read the /etc/fstab file for de-
vices and files with an ``sw'' type, and adds all these entries
as swap devices. If no swap devices are configured, swapctl will
exit with an error code.
.
.
.
-tblk|noblk
This flag modifies the function of the -A option. The -t option
allows the type of device to add to be specified. An argument of
blk causes all block devices in /etc/fstab to be added. An argu-
ment of noblk causes all non-block devices in /etc/fstab to be
added. This option is useful in early system startup, where
swapping may be needed before all file systems are available,
such as during disk checks of large file systems.